Removable adhesive of polyepoxide, curing agent and microspheres

ABSTRACT

A thermosetting adhesive comprising a polyepoxide resin, a curing agent such as an imidazole and polymeric or elastomeric microspheres is removable when heated to a temperature greater than the use temperature of from about room temperature to about 185° C.

BACKGROUND OF THE INVENTION

The invention relates to thermosettable adhesive compositions.

Thermosettable adhesive compositions have been used in a variety ofapplications where a semi-structural bond between two substrates isrequired. The semi-structural bond is necessary to ensure that thesubstrates are inseparable. In most applications, the bond is designedto be permanent. There are applications, however, in which it would bepreferable for the adhesive composition to exhibit high performance bondproperties during use (i.e., the period and environmental conditions,e.g., temperature range, over which the adhesive composition performs asa semi-structural adhesive), yet be removable after use. A tensionexists between these opposing performance criteria. In the aerospaceindustry, for example, decorative sheets are often adhered to theinterior walls of aircraft cabins using thermosettable adhesives. Overtime the decorative sheets become marred (e.g., soiled, cut or torn) andstyles change. It would be preferable if these decorative sheets couldbe removed and replaced with new sheets. Following cure, however,substrates bonded together by thermoset adhesive compositions aresubstantially inseparable. As a result, efforts to separate thesubstrates are often unsuccessful and result in substrate damage. Inaddition, the cured adhesive composition exhibits unpredictable cohesiveand adhesive failure at either substrate.

A variety of thermosettable adhesive compositions are used to formsemi-structural bonds to substrates. Thermosettable polyurethaneadhesive compositions are often used to bond substrates together. Singlepackage solvent-borne thermosettable polyurethane adhesive compositionsrely on atmospheric moisture for curing. Water-borne thermosettablepolyurethane adhesive compositions are cured by the addition of waterdispersible isocyanate groups to the adhesive composition. Theisocyanate groups react with the urea, amino-hydrogen and hydroxylgroups present in the water-borne prepolymer to crosslink thecomposition. U.S. Pat. No. 3,765,972 (Wesp) describes apressure-sensitive adhesive composition for use wherever a strongpermanent bond is desired. The adhesive composition includes a latex anda transient tackifier that includes an epoxy resin and a curing agent.The latex portion of the adhesive composition provides the film-formingcapability of the composition.

U.S. Pat. No. 5,464,902 (Recker) describes incorporating minorquantities of functionalized, partially crosslinked, elastomericparticles having a glass transition temperature of less than 10° C. intoepoxy resin systems to toughen the epoxy resin systems againstimpact-induced damage. The toughened matrix resin systems may beutilized as neat films in structural adhesives or may be scrimsupported.

U.S Pat. No. 4,049,483 (Loder et al) describes a hot melt adhesivesystem of hot melt adhesive and inherently tacky elastomeric copolymermicrospheres. The hot melt system has pressure sensitive adhesivecharacteristics at room temperature. The patent further describes addinga tackifying agent to the hot melt adhesive system to enhance the roomtemperature adhesion of the adhesive surface. Once the hot-melt adhesivesystem has been heat-activated, the adhesive is capable of forming asubstantially permanent high strength hot melt bond. The basicproperties of the hot melt matrix are unaffected by the inclusiontherein of the microspherical adhesive.

SUMMARY OF THE INVENTION

In a first aspect, the invention features a thermosettable adhesivecomposition that includes a polyepoxide resin, a curing agent, and aplurality of microspheres. The microspheres, the polyepoxide resin, andthe curing agent, and the relative amounts thereof, are selected suchthat upon cure (i.e., a sufficient degree of crosslinking to achieve asemi-structural bond to a substrate) the composition is capable offorming a semi-structural bond to a substrate and is cleanly thermallyremovable from the substrate. The adhesive composition can also includeup to about 20% by weight flame retardant.

The cured adhesive compositions preferably exhibit a peel adhesionstrength of at least 3.5 N/cm (2 pounds per inch width (“piw”)), morepreferably at least 10.5 N/cm (6 piw), when measured on an abradedphenolic resin impregnated fiberglass substrate or a polycarbonatesubstrate at room temperature (about 20° C. to 25° C.). In addition,preferred adhesive compositions exhibit no greater than about 35%retention (more preferably no greater than about 20% retention) ofinitial peel adhesion strength at a temperature greater than the upperuse temperature. The adhesive compositions preferably exhibit a peeladhesion strength of at least 3.5 N/cm (2 piw) measured at roomtemperature and no greater than about 35% retention of initial peeladhesion strength at a temperature greater than the upper usetemperature.

In one preferred embodiment, the cured composition exhibits no greaterthan about 35% retention of initial peel adhesion strength at atemperature greater than about 50° C., more preferably the compositionexhibits no greater than about 35% retention of initial peel adhesionstrength at a temperature of about 15° C. greater than the upper usetemperature.

Preferred adhesive compositions have a ratio of weight of polyepoxideresin to weight of microspheres of between about 70:30 and about 35:65.The adhesive composition is preferably dispersed in water. The adhesivecomposition may be tacky or tack-free prior to cure.

The adhesive composition preferably cures at a temperature greater thanroom temperature, more preferably between about room temperature andabout 200° C.

Preferred microspheres are characterized as having a surface that isessentially free of functional groups capable of reacting with thepolyepoxide resin. The microspheres can be tacky or tack-free, solid oryhollow microspheres. Preferred microspheres include tacky, solidmicrospheres. The microspheres preferably have an average diameterbetween about 1 micrometer and about 20 micrometers. The microspherespreferably include the reaction product of isooctyl acrylate, acrylicacid and poly(ethylene oxide)acrylate.

The curing agents preferably include heat-activated curing agents orphotolytically-activatedcuring agents. The curing agents can include ablend of an epoxy homopolymerization catalyst (e.g., tertiary amines,imidazoles, substituted derivatives of imidazoles and combinationsthereof) and an addition curing agent (e.g., primary and secondaryamines).

In another embodiment, the invention features an article that includes asubstrate having a surface, at least a portion of which is provided withthe above-described thermosettable adhesive composition. The substratecan be a rigid substrate or a flexible substrate, e.g., a film. Thearticle may further include a second substrate in contact with theadhesive composition. The adhesive composition of the article, uponcure, preferably exhibits a peel adhesion of at least about 3.5 N/cmmeasured on the substrate at room temperature, and no greater than about35% retention of initial peel adhesion strength at a temperature greaterthan the upper use temperature.

In another aspect, the invention features a method for making an articlethat includes providing the above-described adhesive composition on atleast a portion of a substrate (e.g., a more rigid substrate). Themethod can further include contacting the adhesive composition with asecond substrate (e.g., a more flexible substrate). The methodpreferably also includes curing the adhesive composition.

In one preferred embodiment, the invention features a method for makingan adhesive article that includes contacting a surface with an articlethat includes the above-described thermosettable adhesive compositionand heating the composition to a temperature sufficient to cure thecomposition. The method may further include contacting a substrate withthe adhesive composition before heating the composition.

In other embodiments, the invention features a method for removing anarticle from a surface that includes heating an article having a curedthermosettable adhesive composition disposed between a first substrateand a second substrate and forming a semi-structural bond between thesubstrates, to a temperature greater than the use temperature of thecomposition, and cleanly removing the composition from one of thesubstrates.

In another aspect, the invention features a method for making athermosettable adhesive composition cleanly thermally removable byincorporating a plurality of microspheres therein. The thermosettableadhesive composition, upon cure, is capable of forming a semi-structuralbond to a substrate.

The invention provides adhesive compositions that are dispersed inwater. As a result, they can be applied to surfaces as a water-basedsystem as opposed to a solvent based system, which substantially reducesvolatile organic compound emissions during application of the adhesivecomposition. The adhesive compositions are also relatively fast dryingand stable to variations in temperature, pH, metal ion concentration,and shear force.

The adhesive compositions promote selective adhesive failure by allowingtwo surfaces to be inseparable at use temperature, yet readily separableupon heating such that a substantial portion of the adhesive compositionwill remain adhered to one of the two surfaces. This feature isparticularly useful in decorative laminate applications because itallows for the simultaneous removal of substantially the entiredecorative laminate and substantially all of the adhesive compositionassociated with the portion of the decorative laminate that is beingremoved.

Other advantages and features of the invention will be apparent from thedetailed description and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The thermosettable adhesive compositions include a polyepoxide resin, acuring agent for the polyepoxide resin, a plurality of microspheres, andoptionally a flame retardant. The adhesive compositions, upon cure,yield thermoset adhesive compositions that are capable of forming asemi-structural bond between two substrates and, when heated to atemperature greater than the use temperature (i.e., the temperature ortemperature range over which the adhesive composition performs as asemi-structural adhesive), exhibit selective adhesive failure, i.e.,adhesive failure at one substrate, upon peel. The adhesive compositionis preferably utilized in an article having a more flexible substratebonded to a more rigid substrate. When such an article is heated to atemperature greater than the use temperature of the adhesivecomposition, the more flexible substrate can be peeled back and awayfrom the more rigid substrate. As the substrates are peeled apart,adhesive failure occurs at one of the substrates, preferably the morerigid substrate. As a result, the cured adhesive composition can besimultaneously removed with one of the substrates, preferably the moreflexible substrate.

Although preferred articles include a more flexible substrate and a morerigid substrate, the substrates generally can be made from any materialsuitable for the particular application envisioned for the substrate.Suitable materials for more flexible substrates include, e.g., polymericfilm, metallic foil, paper, cloth, silk, woven fabrics, nonwovenfabrics, wood veneers, and leather. Appropriate materials for polymericfilms include cellulose acetate film, ethyl cellulose film, polyolefins,polystyrene, polyvinyl alcohol, polyesters (e.g., polyethyleneterephthalate and polybutylene terephthalate), poly(caprolactam),poly(vinyl fluoride), and the like.

Suitable materials for more rigid substrates include, e.g., thosematerials suitable for the more flexible substrate in a form that ismore rigid than the more flexible substrate, (e.g., the rigid substratewould have a greater thickness than the thickness of the more flexiblesubstrate), cellulosic materials (e.g., wood and wood products), metal,plastic, ceramic and the like.

The adhesive compositions are semi-structural adhesive compositions andmay be structural adhesive compositions. Semi-structural adhesivecompositions are thermosettable adhesive compositions that exhibitsemi-structural bond strengths and heat resistance up to the upper usetemperature of the adhesive compositions (i.e., the highest temperatureat which the adhesive composition performs as a semi-structuraladhesive)upon cure. Semi-structural bond strengths may exceed thebreaking strength of one or both substrates joined by the adhesivecomposition. In addition, semi-structural adhesive compositions possessenvironmental resistance, e.g., resistance to humidity, heat aging,corrosion, and solvents. Semi-structural adhesive compositions are usedto bond the non-load bearing parts of a product and are often used inthe automobile and aerospace industries to bond, e.g., laminates such asvinyl foam to hard thermoplastics and interior trim constructions (e.g.,fabric to fiber board and veneer to particle board).

The peel adhesion strength of an adhesive composition may vary dependingupon the parameters of the system in which the adhesive composition isemployed. These parameters include, for example, peel rate, angle ofpeel, temperature, humidity, and the nature and surface properties ofthe substrates that are bonded together by the adhesive composition.Peel adhesion strengths of systems that employ semi-structural adhesivecompositions ultimately depend upon the application but are typified byroom temperature peel adhesion values of at least 3.5 N/cm (2 piw), morepreferably at least 10.5 N/cm (6 piw), most preferably at least 17.5N/cm (10 piw), although these values could be higher. Measurement ofpeel adhesion strength is described below.

The polyepoxide resin, the curing agent and the microspheres and theamounts thereof are selected such that a majority of the cured adhesivecomposition is cleanly removable from a substrate upon heating to atemperature greater than the upper use temperature. Clean removabilityexists when at least a majority of the cured adhesive compositionexhibits adhesive failure at a first of two substrates and a minoramount of the adhesive composition exhibits cohesive failure or adhesivefailure at the second of the two substrates. The adhesive residueremaining on the second substrate is easily removable, e.g., by lightrubbing with fingers, scraping with a fingernail, with a mild scouringpad and light hand pressure, liquid cleaners, and other like methods.

Removability is exemplified in the Examples set forth herein.Removability of the cured adhesive compositions may be expressed aspercent retention of initial peel adhesion strength. The percentretention of initial peel adhesion strength is a ratio, expressed inpercent, of 1) peel adhesion strength measured at a temperature otherthan room temperature according to a test procedure, and 2) peeladhesion strength measured at room temperature according to the sametest procedure. The samples tested must have the same environmentalhistory. Preferred adhesive compositions exhibit no greater than about35% retention of initial peel adhesion strength, preferably no greaterthan about 25% retention of initial peel adhesion strength, morepreferably no greater an about 20% retention of initial peel adhesionstrength, and most preferably no greater than about 15% retention ofinitial peel adhesion strength when measured at a temperature greaterthan the use temperature.

The cured adhesive compositions preferably exhibit adhesive failure andare cleanly removable from a substrate when heated to a temperaturegreater than the upper use temperature of the adhesive composition,preferably at least about 50° C. greater than the upper use temperature,more preferably at least about 25° C. greater than the upper usetemperature.

Preferred adhesive compositions have a ratio of weight of polyepoxideresin to weight of microspheres, on a dry weight basis, in the range ofabout 70:30 to about 35:65, more preferably in the range of about 65:35to about 45:55, most preferably in the range of about 65:35 to about55:45. Depending upon the application, the polyepoxide resins, curingagents for the polyepoxide resin, and microspheres and the relativeamounts thereof can be selected so as to achieve adhesive compositionsthat, upon cure, exhibit semi-structural bond properties and cleanthermal removability. The amount of microspheres, in particular, can beadjusted so as to provide adhesive compositions that, upon cure, havethe properties set forth above.

The adhesive compositions will now be described in greater detail.

Polyepoxide Resin

Polyepoxide resins useful in the compositions of the invention are thosecompounds containing at least two 1,2-epoxide groups, i.e., groupshaving the following structure

polymerizable by a ring opening reaction. Such materials, broadly calledpolyepoxides, include both monomeric and polymeric polyepoxides and canbe aliphatic, cycloaliphatic, and aromatic and blends thereof. Aromaticpolyepoxides are preferred because they can impart elevated temperatureperformance properties (e.g., high glass transition temperature andmaintenance of peel adhesion strength at elevated temperatures) to thecured adhesive composition and can impart structural properties thereto.These materials generally have, on the average, a functionality of twoto four, more preferably two to three, and most preferably about two.The polyepoxide resin preferably has a molecular weight of between about250 and about 5000, more preferably between about 250 and about 2000,most preferably between about 500 and 1000, and an epoxide equivalentweight of about 60 to about 2500 grams per equivalent. The “epoxideequivalent weight” of a given compound is defined as the molecularweight divided by the number of epoxide groups in the compound.

Examples of suitable aromatic polyepoxide resins include polyglycidylethers of polyhydric phenols, eg., pyrocatechol, resorcinol,hydroquinone, 4,4′-dihydroxydiphenyl methane,4,4′-dihydroxydiphenyldimethylmethane,4,4′-dihydroxy-3,3′-dimethyldiphenylmethane,4,4′-dihydroxydiphenylmethylmethane, 4,4′-dihydroxydiphenylcyclohexane,4,4′-dihydroxy-3,3′dimethyldiphenyl dimethylmethane,4,4′-dihydroxydiphenylsulfone, andtris-(4-hydroxyphenyl)methane);9,9-bis(4-hydroxyphenyl)fluorene andortho-substituted analogs thereof such as disclosed in U.S. Pat. No.4,707,534 (Schultz); polyglycidyl ethers of novolacs (i.e., reactionproducts of monohydric or polyhydric phenols with aldehydes,formaldehyde in particular, in the presence of acid catalysts); and thepolyglycidyl ethers of halogenation (e.g., chlorination and bromination)products of the above-mentioned polyvalent phenols.

Other suitable polyepoxide resins include polyglycidyl derivatives ofaromatic amines, i.e., glycidylamines, obtained from the reactionbetween aromatic amines and an epihalohydrin. Examples of suitablepolyglycidyl aromatic amines include N,N′-diglycidyl aniline,N,N′-dimethyl-N,N′-diglycidyl-4,4′-diaminodiphenyl methane,N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenyethane,N,N-diglycidylnaphthalenamine,N,N,N′,N′-tetraglycidyl-1,4-bis[α-(4-aminophenyl)α-methylethyl]bnzene,andN,N,N′,N′-tetraglycidyl-1,4-bis[α-(4-amino-3,5-dimethylphenyl)α-methylethyl]benzene.An example of a suitable polyglycidyl derivative of aromaticaminophenols is glycidylamino-glycidyloxybenzene, as disclosed in U.S.Pat. No. 2,951,825 (Reinking et al.). Other suitable polyepoxide resinsinclude, e.g., polyglycidyl esters of aromatic polycarboxylic acids,e.g., diglycidyl esters of phthalic acid, isophthalic acid andterephthalic acid.

Preferred aromatic polyepoxide resins include polyglycidyl ethers ofnovolacs, diglycidyl ether of 4,4′-dihydroxy diphenyldimethyl methaneand diglycidyl ether of 4,4′-dihydroxydipbenylmethane. Many suitablearomatic polyepoxide resins are available conmmercially includingMY™-720 (from Ciba Specialty Chemicals Corporation, Brewster, N.Y.),ERL™-0510 (from Ciba Specialty Chemicals Corporation), the EPON™ seriesof materials from Shell Chemical Co., Houston, T.X. (e.g., EPON™HPT-1071, EPON™ HPT-1072, EPON™ HPT-1079, and EPON™ 828), and theD.E.R.™, D.E.N.™ and QUATREX™ families of materials from Dow ChemicalCompany, Midland, Mich. (e.g., D.E.R™ 332, D.E.R.™ 661, D.E.N.™ 438, andQUATREX™ 1010).

Examples of suitable aliphatic polyepoxide resins include polyglycidylethers of aliphatic polyols, e.g., glycerol and hydrogenated4,4′-dihydroxydiphenyl-dimethylmethane, and cycloaliphatic polyepoxideresins.

Minor amounts of monofunctional polyepoxide resins may also be used incombination with the polyepoxide resins to impart improved surfacewetting characteristicsto the adhesive composition before cure andflexibility to the cured adhesive composition.

The polyepoxide resins are preferably in the form of an aqueousdispersions. Methods of preparing polyepoxide resin dispersions areknown to the skilled artisan.

Examples of commercially available aqueous dispersions of aromaticpolyepoxide resins that are suitable for use in the adhesivecompositions of the invention include the EPI-REZ® series availableunder the trade designationsWD-510,3520-WY-55,5522-WY-55,3522-WY-60,3540-WY-55, from Shell, ECN1400, PY 323, PZ 3901, PZ 3907, PZ 3917, XU 3900, and XU 3903 (availableunder the trade designation ARALDITE from Ciba Specialty ChemicalsCorporation).

Curing Agent

The term “curing agent” is used broadly to include not only thosematerials that are conventionally regardedas curing agents but alsothose materials that catalyze epoxy polymerization as well as thosematerials that may act as both curing agent and catalyst. Preferredcuring agents for the polyepoxide resin include, e.g., room temperaturecuring agents, heat-activated curing agents and combinations thereof,and photolytically activated curing agents. Room temperature curingagents and heat-activated curing agents, can include, e.g., blends ofepoxy homopolymerization catalyst type curing agents and addition typecuring agents. The curing agents preferably react at temperatures ofbetween about room temperature and about 200° C., more preferably about30° C. and 150° C., most preferably between about 50° C. and about 115°C. Preferred curing agents are water soluble or water dispersible.

Examples of suitable curing agents include polybasic acids and theiranhydrides, e.g., di-, tri- and higher carboxylic acids such as oxalicacid, phthalic acid, terephthalic acid, succinic acid, alkyl and alkenylsubstituted succinic acids, tartaric acid, and anhydrides, e.g.,phthalic anhydride, succinic anhydride, maleic anhydride, nadicanhydride and pyromellitic anhydride; polymerizable unsaturated acids,e.g., those containing at least 10 carbon atoms, e.g., dodecendioicacid, 10, 12-eicosadiendioic acid; and mercaptans.

Examples of other suitable curing agents include nitrogen containingcompounds, e.g., benzyldimethylamine, benzylamine, N,N-diethyl aniline,melamine, pyridine, hydrazides, and aromatic polyamines, such as o-, m-,and p-phenylene diamine, 4,4′-diaminodiphenylsulfone,3,3′-diaminodiphenylsulfone, and 4,4′diaminodiphenylsulfide,4,4′-diaminodiphenylketone, 4,4′-diaminodiphenyl ether,4,4′-diaminodiphenylmethane 1,3-propanediol-bis(4-aminobenzoate),fluorene-containingamines (e.g., 9,9-bis(4-aminophenyl)fluorene,9,9-bis(3-methyl-4-aminophenyl)fluorene,9,9-bis(3,5-dimethyl-4-methylaminophenyl)fluorene,9,9-bis(3,5-dimethyl-4-aminophenyl)fluorene,9,9-bis(3,5-diisopropyl-4-aminophenyl)fluorene, and9,9-bis(3-chloro-4-aminophenyl)fluorene)1,4-bis[α-(4-aminophenyl)α-methylethyl]benzene,1,4-bis[α-(4-amino-3,5-dimethylphenyl)-α-methylethyl]benzene,bis(4-amino-3-methylphenyl)sulfone,1,1′-biphenyl-3,3′-dimethyl-4,4′-diamine,1,1′-biphenyl-3,3′-dimethoxy-4,4′-diamine and diaminonaphthalenes.

Preferred curing agent include, e.g., aliphatic nitrogen-containingcompounds, including poly(ether) amines, guanidines (e.g., dicyandiamideand tetramethyl guanidine), imidazoles (e.g.,2-ethyl-4-methylimidazole), cyclohexylamine, diethylenetriamine,triethylenetetraamine, cyclohexyldiamine, tetramethylpiperamine,N,N-dibutyl-1,3-propanediamine, N,N-diethyl-1,3-propane diamine,1,2-diamino-2-methyl-propane, 2,3-diamino-2-methylbutane,2,3-diamino-2-methylpentane, and 2,4-diamino-2,6-dimethyloctane.

Examples of suitable phenolic curing agents include polyhydric phenols,e.g., pyrocatechol, resorcinol, hydroquinone,4,4′-dihydroxydiphenylmethane, 4,4′-dihydroxydiphenyidimethylmethane,4,4′-dihydroxy-3,3′-dimethyldiphenyknethane,4,4′-dihydroxydiphenylmethylmethane, 4,4′dihydroxydiphenylcyclohexane,4,4′-dihydroxy-3,3′-dimethyldiphenyldimethylmethane,4,4′-dihydroxydiphenylsulfone, and tris-(4-nydroxyphenyl)methane;and9,9-bis(4-hydroxyphenyl)fluorene and ortho-substituted analogs thereof.

Other useful curing agents include chloro-, bromo-, andfluoro-containing Lewis acids of aluminum, boron, antimony, andtitanium, such as aluminum trichloride, aluminum tribromide, borontrifluoride, antimony pentafluoride, titanium tetrafluoride, and thelike. It is also desirable at times that these Lewis acids be blocked toincrease the latency of adhesive compositions containing them.Representative of blocked Lewis acids are BF₃-monoethylamine, and theadducts of HSbF₅X, in which X is a halogen, —OH, or —OR¹ in which R¹ isthe residue of an aliphatic or aromatic alcohol, aniline, or aderivative thereof, as described in U.S. Pat. No. 4,503,211.

Suitable photolytically activated curing agents include, e.g., iodoniumand sulfonium salts of antimony and cobalt, and bis(arene) ironcompounds.

Examples of commercially available curing agents suitable for use in theadhesive compositions include EPI-CURE™ 8535-W-50 and EPI-CURE™8537-WY-60 (available from Shell Chemical Co., Houston, Tex.), HY 955(available from Ciba Specialty Chemicals Corporation), AMICURE™ CG-1400,ANCAMINE™ 2337S, CUREZOL® 2E4MZ, and CUREZOL® PHZ-S (available from AirProducts, Pacific Anchor Chemical, Allentown, Pa.), and DCA-221(available from Dixie Chemical Company, Pasadena, Tex.).

The curing agent is preferably present in an amount of about 2 to about110 parts by weight, per 100 parts by weight of the polyepoxide resin.When the curing agent is a carboxylic acid, a guanidine, a phenol, ananhydride, or a primary or secondary amine, the curing agent preferablyis present in about 0.5 to about 1.7 equivalents of acid, anhydride, oramine per equivalent of epoxide group. When the curing agent is ananhydride or a phenol, accelerators may be added in amounts of about 0.1to about 5.0% based on the weight of polyepoxide resin. Accelerators mayalso be used alone and in the amounts noted. Examples of suitableaccelerators include aromatic tertiary amines such as benzyldimethylamine, and imidazoles such as 2-ethyl-4-methylimidazole. Lewis acid arepreferably used in amounts of between about 0.1 and about 5% by weightbased on the total weight of the polyepoxide resin.

Microspheres

The microspheres exist within the adhesive composition as adiscontinuous phase and are characterized such that any functionalgroups present on the surface of the microspheres are incapable ofreacting with or dissolving in the polyepoxide resin. It is well knownthat carboxylic acids and amines will react with polyepoxide resins.However, when such components are components of a microsphere and ahydrophilic macromer is also included as a component of the microsphere,a reaction can be avoided. Not wishing to be bound by theory, it isbelieved that the hydrophilic macromer located at the surface of themicrosphere sterically shields the functional groups present on thesurface and prevent them from reacting with the polyepoxide resin.

The microspheres are polymeric, elastomeric, solvent insoluble andsolvent dispersible. In addition, they can be solid or hollow and tackyor tack-free. The specific type of microsphere can be selected to yieldthe desired properties of the adhesive composition for the particularapplication. Polymer microspheres preferably are formed by free radicalsuspension polymerization.

The microspheres generally will have an average diameter between about 1micrometer (μm) and 250 μm, preferably between about 1 μm and 75 μm,more preferably between about 1 μm and 20 μm, and most preferablybetween about 1 μm and 10 μm. When the microspheres are hollow, thevoids typically range in size from less than about 1 μm up to about 100μm or larger.

For the formation of tacky microspheres preferred monomers include alkylacrylates and methacrylates. These monomers are monofunctionalunsaturated acrylate and methacrylate esters of non-tertiary alkylalcohols. The alkyl groups of these alcohols preferably contain from 4to 14 carbon atoms. These acrylate monomers are oleophilic, wateremulsifiable, have restricted water solubility, and as homopolymers,generally have glass transition temperatures below about −10° C.Examples of such monomers include, but are not limited to, isooctylacrylate, 4-methyl-2-pentyl acrylate, 2-methylbutyl acrylate, isoamylacrylate sec-butyl acrylate. n-butyl acrylate, 2-ethylhexyl acrylate,isodecyl methacrylate, isononyl acrylate, isodecyl acrylate, andmixtures thereof.

Preferred acrylate monomers include isooctyl acrylate, isononylacrylate, isoamyl acrylate, isodecyl acrylate, 2-ethylhexyl acrylate,n-butyl acrylate, sec-butyl acrylate, and mixtures thereof

Vinyl ester monomers suitable for use in the microparticles include, butare not limited to, vinyl 2-ethylhexanoate, vinyl caprate, vinyllaurate, vinyl perlargonate, vinyl hexanoate, vinyl propionate, vinyldecanoate, vinyl octanoate, and other monofunctional unsaturated vinylesters of linear or branched carboxylic acids comprising 1 to 14 carbonatoms that, as homopolymers, have glass transition temperatures belowabout −10° C. Preferred vinyl ester monomers include vinyl laurate,vinyl caprate, vinyl 2-ethylhexanoate, and mixtures thereof.

Acrylate or methacrylate or other vinyl monomers that, as homopolymers,have glass transition temperatures higher than about −10° C. to 0° C.,e.g., tert-butyl acrylate, isobornyl acrylate, butyl methacrylate, vinylacetate, acrylonitrile, mixtures there of, and the like, may optionallybe utilized in conjunction with one or more of the acrylate,methacrylate and vinyl ester monomers, provided the glass transitiontemperature of the resultant polymer is below about −10° C.

For the production of elastomeric microspheres, other suitableco-monomers include polar co-monomers, e.g., monoolefinic monocarboxylicacids, monoolefinic dicarboxylic acids, acrylamides, N-substitutedacrylamides, salts thereof, and mixtures thereof. Specific examplesinclude acrylic acid, methacrylic acid, itaconic acid, crotonic acid,maleic acid, fumaric acid, sulfoethyl methacrylate, and ionic monomerssuch as sodium methacrylate, ammonium acrylate, sodium acrylate,trimethylaminep-vinyl benzamide,4,4,9-trimethyl-4-azonia-7-oxo-8-oxa-dec-9-ene-1-sulphonate,N,N-dimethyl-N-(beta-methoxy-ethyl)ammonium propionate betaine,trimethylamine methacrylamide, 1,1-dimethyl-1-(2,3-dihydroxypropyl)aminemethacrylamide, N-vinyl pyrrolidone, N-vinyl caprolactam, acrylamide,t-butyl acrylamide, dimethyl amino ethyl acrylamide, N-octyl acrylamide,mixtures thereof, and the like. Preferred polar monomers includemonoolefinic monocarboxylic acids, monoolefinic dicarboxylic acids,acrylamides, N-substituted acrylamides, salts thereof and mixturesthereof. Examples of such monomers include but are not limited toacrylic acid, sodium acrylate, N-vinyl pyrrolidone, and mixturesthereof.

Optionally, free radically reactive hydrophilic macromers or polymersmay be included and can be used as co-monomers to produce microsphereswith pendent hydrophilic moieties. The hydrophilic components can act ascrosslinkers when they are multi-functional. Preferred are freeradically reactive hydrophilic oligomers (a polymer having a low numberof repeating units, generally 2 to 20) and/or polymers includingpoly(alkylene oxides) (e.g., poly(ethylene oxide)), poly(vinyl methylether), cellulose derivatives and mixtures thereof.

Other suitable hydrophilic co-monomers include macromonomers, e.g.,acrylate terminated poly(ethylene oxide), methacrylate terminatedpoly(ethylene oxide), methoxy poly(ethylene oxide) methacrylate, butoxypoly(ethylene oxide) methacrylate, p-vinyl benzyl terminatedpoly(ethylene oxide), acrylate terminated poly(ethylene glycol),methacrylate terminated poly(ethylene glycol), methoxy poly(ethyleneglycol) methacrylate, butoxy poly(ethylene glycol) methacrylate, p-vinylbenzyl terminated poly(ethylene glycol), poly(ethylene oxide)diacrylate, poly(ethylene oxide) dimethacrylate, and mixtures thereof.These functionalized materials are preferred because they are easilyprepared through well-known ionic polymerization techniques and are alsohighly effective in providing grafted hydrophilic segments along freeradically polymerized microsphere polymer backbones.

The composition from which the elastomeric microspheres of the inventionare made may also contain a multifunctional crosslinking agent. The term“multifunctional” as used herein refers to crosslinking agents thatpossess two or more free radically polymerizable ethylenicallyunsaturated groups. Useful multifunctional crosslinking agents includeacrylic or methacrylic esters of diols such as butanediol diacrylate,triols such as glycerol, and tetraols such as pentaerythritol. Otheruseful crosslinking agents include polymeric multifunctional(meth)acrylates, e.g., poly(ethylene oxide) diacrylate or poly(ethylene)oxide dimethacrylate; polyvinylic crosslinking agents, such assubstituted and unsubstituted divinylbenzene; and difunctional urethaneacrylates, such as “EBECRYL” 270 and “EBECRYL” 230 (1500 weight averagemolecular weight and 5000 weight average molecular weight acrylatedurethanes, respectively-both available from UCB Radcure, Inc., Smyrna,Ga.), and mixtures thereof.

When a crosslinker is employed, it is typically employed at a level ofup to about 10 equivalent weight percent. Above about 0.15 equivalentweight percent, based on the total polymerizable microspherecomposition, most elastomeric microspheres become tack-free. The“equivalent weight percent” of a given compound is defined as the numberof equivalents of that compound divided by the total number ofequivalents in the total (microsphere) composition, where an equivalentis the number of grams divided by the equivalent weight. The equivalentweight is defined as the molecular weight divided by the number ofpolymerizable groups in the monomer (in the case of those monomers withonly one polymerizable group, equivalent weight=molecular weight). Thecrosslinker can be added at any time before 100% conversion to polymerof the monomers of the microsphere composition. Preferably, crosslinkeris added before initiation occurs.

The elastomeric microspheres preferably comprise between about 45 andabout 100 parts of at least one free radically polymerizable monomer,optionally up to about 15 parts of one or more polar monomers, and about0 to about 40 parts of at least one hydrophilic component.

More preferably, the elastomeric microspheres comprise about 80 to about100 parts, most preferably 90 to 100 parts, of one or more freeradically polymerizable monomers selected from the group consisting ofalkyl acrylate esters, alkyl methacrylate esters, vinyl esters, andmixtures thereof where the alkyl group is a C₄ to C₁₂ alky, optionallyup to about 10 parts of at least one polar monomer, and optionally up toabout 10 parts of a hydrophilic component. Most preferably themicrospheres comprise about 95 to about 99 parts of the free radicallypolymerizable monomers, about 0.5 to about 5.0 parts of a hydrophiliccomponent and, optionally about 0.1 to about 5.0 parts of a polarmonomer.

The relative amounts of the components are important to the propertiesof the resultant microspheres and the adhesive composition as a whole.

If hollow, elastomeric microspheres are desired, they may be obtainedvia a “two-step” process comprising the steps of:

(a) forming a water-in-oil emulsion by mixing (1) an aqueous solution(which may contain some of the hydrophilic component and/or some of theoptional polar monomer) with (2) oil phase base monomers, a free radicalpolymerization initiator, and internal crosslinking agent (if any isused);

(b) forming a water-in-oil-in-wateremulsion by dispersing thewater-in-oil emulsion from step (a) into an aqueous phase (containingany of the hydrophilic component and/or polar monomer not added in step(a)); and

(c) initiating suspension polymerization, usually by applying heat(preferably about 40 to 60° C., more preferably about 50 to 60° C.) orradiation (e.g., ultraviolet radiation).

Emulsifiers having a low hydrophilic-lipophilc balance (HLB) value areused to facilitate the formation (usually by agitation) of thewater-in-oil emulsion in the first step. Suitable emulsifiersare thosehaving an HLB value below about 7, preferably in the range of about 2 to7. Examples of such emulsifiers include sorbitan monooleate, sorbitantrioleate, and ethoxylated oleyl alcohol such as Brij™ 93, availablefrom Atlas Chemical Industries, Inc. A thickening agent, e.g., methylcellulose, may also be included in the aqueous phase of the water-in-oilemulsion.

The aqueous phase into which the water-in-oil emulsion is dispersed instep (b) contains an emulsifier having an HLB value above about 7.Examples of such emulsifiers include ethoxylated sorbitan monooleate,ethoxylated lauryl alcohol, and alkyl sulfates. The emulsifierconcentration (for both steps (a) and (b)) should be greater than itscritical micelle concentration, which refers to the minimumconcentration of emulsifier necessary for the formation of micelles,i.e., submicroscopic aggregations of emulsifier molecules. Criticalmicelle concentration is slightly different for each emulsifier, usableconcentrations ranging from about 1.0 ×10⁻⁴ to about 3.0 moles/liter.Additional detail concerning the preparation ofwater-in-oil-in-wateremulsions, i.e. multiple emulsions, may be found invarious literature references, e.g., Surfactant Systems: TheirChemistry, Pharmacy, & Biology, (D. Attwood and A.T. Florence, Chapman &Hall Limited, New York, 1983).

Useful initiators are those which are normally suitable for free radicalpolymerization of acrylate or vinyl ester monomers and which are oilsoluble and of very low solubility in water, typically less than 1 g/100g water at 20° C. Examples of such initiators include azo compounds,hydroperoxides, peroxides, and the like, and photoinitiators such asbenzophenone, benzoin ethyl ether, 2,2-dimethoxy-2-phenyl acetophenone.The initiator is generally used in an amount ranging from about 0.01% upto about 10% by weight of the total polymerizable composition,preferably up to about 5%.

Use of a substantially water soluble polymerization initiator, such asthose generally used in emulsion polymerization, causes the formation ofsubstantial amounts of latex. During suspension polymerization, anysignificant formation of latex is undesirable because of the extremelysmall particle size.

Hollow microspheres may also be prepared by a simpler “one-step” processcomprising aqueous suspension polymerization of the hydrophiliccomponent, the base monomer, and the polar monomer (which is notoptional for this process) in the presence of an emulsifier which iscapable of producing, inside the droplets, a water-in-oil emulsion thatis substantially stable during both formation of the emulsion andsubsequent suspension polymerization.

Useful emulsifiers are anionic materials having an HLB value greaterthan 25 and include alkylaryl ether sulfates such as sodium alkylarylether sulfate, e.g., Triton™ W/30, available from Rohm and Haas;alkylaryl poly(ether) sulfates such as alkylaryl poly(ethylene oxide)sulfates, preferably those having up to about 4 ethoxy repeat units; andalkyl sulfates, such as sodium lauryl sulfate, and sodium hexadecylsulfate, triethanolamine lauryl sulfate, and sodium hexadecyl sulfate;alkyl poly(ether) sulfates, such as alkyl poly(ethylene oxide) sulfates,preferably those having up to about 4 ethoxy units. Alkyl sulfates,alkyl ether sulfates, alkylaryl ether sulfates, and mixtures thereof arepreferred.

Non-ionic emulsifiers having an HLB value of between about 13 and 25 canbe utilized in conjunction with the anionic emulsifiers. Examples ofnon-ionic emulsifiers include Siponic™ Y-500-70 (ethoxylated oleylalcohol, available from Alcolac, Inc.), PLURONIC® P103, and Tween™ -40(from ICI America). As in the two-step process, the emulsifier isutilized in a concentration greater than its critical micelleconcentration. Polymeric stabilizers may also be present but are notnecessary.

The above-described one-step method may be varied by combining the basemonomer with non-ionic emulsifiers, oil soluble polymerizationinitiator, and any multifunctional internal crosslinker before the basemonomer is added to the aqueous phase containing a hydrophiliccomponent, emulsifier and any optional polar monomer. (The polar monomeris optional for this process.) The resulting emulsion is suspensionpolymerized to yield hollow microspheres. Anionic emulsifiers with anHLB value greater than 7 may be included in the aqueous phase tostabilize the system during suspension polymerization but are notrequired.

Solid microspheres may be prepared via the suspension polymerizationmethods disclosed in U.S. Pat. Nos. 3,691,140, 4,166,152, and 4,636,432.In general, these suspension polymerization techniques use ionic ornon-ionic emulsifiers in an amount greater than the critical micelleconcentration and/or protective colloids, finely divided inorganicsolids, or the like.

Each suspension polymerization method (whether producing hollow or solidmicrospheres) may be modified by withholding the addition of all or someof the hydrophilic component and/or any optional polar monomer untilafter polymerization of the oil phase base monomer has been initiated.In this instance, however, these components must be added to thepolymerizing mixture prior to 100% conversion of the base monomer.Similarly, the internal crosslinker (if used) can be added at any timebefore 100% conversion to polymer of the monomers of the microspherecomposition. Preferably it is added before initiation occurs. Thehydrophilic component can be added to the oil or water phase in thefirst step or the water phase in the second step, either before or afterpolymerization is initiated, or some combination of these options.

Following polymerization, an aqueous suspension of the hollow or solidmicrospheres is obtained which is stable to agglomeration or coagulationunder room temperature conditions (i.e., about 20 to about 25° C.). Thesuspension may have a non-volatile solids content of from about 10 toabout 60 percent by weight.

Other Additives

Other additives that can be blended into the thermosettable adhesivecompositions to alter the characteristics of either the uncured or curedcompositions include, but are not limited to, flame retardants,thickening agents, plasticizing agents, antioxidants, pigments,inorganic fillers such as silica and calcium carbonate, clays such asbentonite, reinforcing materials, such as hollow glass microspheres,glass beads, glass fibers, and polymeric fibers. These additives arepreferably added in amounts of less than 10% by weight.

Flame retardants are useful for retarding the spread of fire and smokeemission. Preferred flame retardants are water soluble or waterdispersible and resistant to hydrolysis. Examples of suitable flameretardants include melamine cyanurate, phosphate esters, phosphonateesters, melamine phosphates, ammonium phosphates, phosphazenes, redphosphorus and combinations thereof. Useful commercially available flameretardants include ANTIBLAZE® 19, ANTIBLAZE® 195, ANTIBLAZE® 1045,ANTIBLAZE® N, AMGARD® MC, AMGARD® NH and AMGARD® CHT (all available fromAlbright & Wilson Americas, Inc., Glen Allen, Va.). Flame retardants maybe used in amounts of from 0 to about 20% by weight based on the totalweight of the adhesive composition.

Thickening agents can be used to alter the viscosity characteristics ofthe adhesive compositions to achieve suitable applicationcharacteristics.

Plasticizing agents are useful for reducing the viscosity of theadhesive composition and improving the surface wetting characteristicsof the adhesive composition during the curing process. Plasticizingagents are also useful for decreasing the temperature at which the curedadhesive compositions become thermally removable. Examples of suitableplasticizing agents include phosphate esters and phthalate esters.

Preparation of Adhesive Composition

The adhesive compositions are preferably prepared by blending apolyepoxide resin dispersion with the appropriate quantities ofmicrosphere suspension and curing agent using methods known to thoseskilled in the art. The adhesive compositions may be provided in onepart and two part compositions. That is, one part of the adhesivecomposition may include the curing agent without the polyepoxide resinwhile the second part of the adhesive composition may include thepolyepoxide resin without the curing agent. The adhesive compositionsaredispersed in water and may be applied using a water-based system, e.g.,a system that includes water and an optional cosolvent (e.g., alcohols).

The adhesive compositions may be coated onto a substrate using methodsknown in the art, including, e.g., brush coating, spray coating, knifecoating, bar coating, dip coating and roll coating. The adhesivecomposition may then be dried and cured.

The adhesive compositions may cure using methods known in the artincluding, e.g., photolytic radiation, heat and combinations thereof:The adhesive compositions may cure at a single temperature, e.g., roomtemperature, or over a temperature range, preferably between about roomtemperature and about 200° C., more preferably between about 30° C. andabout 150° C., most preferably between about 50° C. and about 115° C.

Suitable use temperatures for the cured adhesive compositions range fromabout room temperature to about 185° C., more preferably from about roomtemperature to about 135° C., most preferably from about roomtemperature to about 100° C.

The cured adhesive compositions preferably are cleanly removable attemperature greater than the upper use temperature of the adhesivecomposition, preferably at least about 50° C. above the upper usetemperature, more preferably at least about 25° C. above the upper usetemperature. In particular, the cured adhesive compositions are cleanlyremovable by heating the adhesive composition to a temperature greaterthan the use temperature. Preferred adhesive compositions are cleanlythermally removable at temperatures greater than about 40° C., morepreferably greater than about 50° C., most preferably greater than about70° C.

The invention will now be further described by way of the followingexamples.

General Preparation of Aqueous Adhesive Dispersions

Method A

An aqueous dispersion of polyepoxide resin, Epi-Rez® 3520-WY-55, wascombined at room temperature with an aqueous suspension of microspheres,prepared previously as described above, in a glass jar with stirring byhand using a wooden tongue depressor until a uniform dispersion wasobtained. This typically required about 1 to 3 minutes of mixing. Aviscous liquid flame retardant additive, ANTIBLAZE® N, was warmed at 71°C. for about 30 minutes in a forced air oven to reduce viscosity andmake it readily pourable. It was then added in a single portion to theaqueous dispersion of polyepoxide resin and microspheres and stirred,using an overhead electric motor equipped with a propeller blade, untilthe flame retardant was completely dissolved. This typically took about5 minutes. Next, the curing agent(s) was added in a single portion andmixed by hand using a wooden tongue depressor until the curing agent(s)was fully dissolved, usually within 1 to 3 minutes. Finally, a viscousliquid thickening agent, Acrysol® SCT-275, was added, in portions, withmixing by hand using a wooden tongue depressor until it had dissolvedand an increase in the viscosity of the composition was visiblyobserved. The resulting opaque, aqueous adhesive dispersion had aviscosity of about 100 to about 2200 centipoise.

Method B

A two part aqueous adhesive was prepared in the following manner. Part Awas prepared by adding two curing agents, each in a single portion, to20% by weight (based on the total amount present in the final adhesivecomposition once parts A and B are combined) of the aqueous microspheresuspension in an epoxy-lined, 0.95 liter steel can using an overhead airmotor equipped with propeller blade, operating at about 200-600 rpm,until the curing agents were fully dissolved. This typically requiredabout 15 minutes of mixing. Next, 20% by weight (based on the totalamount present in the final adhesive composition once parts A and B arecombined) of a viscous liquid thickening agent, Acrysol® SCT-275, wasadded in a single portion with mixing as before.

Part B was prepared by combining Epi-Rez® 3520-WY-55 with the remaining80% by weight (based on the total amount present in the final adhesivecomposition once parts A and B are combined) of the aqueous microspheresuspension in an epoxy-lined, 3.8 liter steel can using the overhead airmotor under the conditions described for Part A until a uniformdispersion was obtained. This typically required about 15 minutes ofmixing. A viscous liquid flame retardant additive, ANTIBLAZE® N, waswarmed at 71° C. for about 30 minutes in a forced air oven to reduceviscosity and make it readily pourable. It was then added in a singleportion to the aqueous dispersion of polyepoxide resin and microspheresand stirred, using the previously described overhead air motor, untilthe flame retardant was completely dissolved. This generally took about15 minutes. Finally, the remaining 80% by weight (based on the totalamount present in the final adhesive composition once parts A and B arecombined) of the Acrysol® SCT-275 thickening agent was added in a singleportion with mixing as before.

The two parts were kept separate until just prior to use. Uponcombination the resulting opaque, aqueous adhesive dispersion had aviscosity of about 100 to about 1000 centipoise as measured using aBrookfield viscometer.

General Preparation of Laminates

Method A

A rigid substrate was coated with the aqueous adhesive dispersion usinga No. 26 Meyer Rod to give a wet film thickness of about 0.152 to about0.178 millimeters (measured). The rigid substrate employed was either 1)a piece of polycarbonate (available under the trade designation Lexan®,from General Electric Co., Schenectady, N.Y.) having a length of about10.2 centimeters, a width of about 5.1 centimeters, and a thickness ofabout 0.32 centimeters, which was used as received after removing theprotective paper cover; or 2) a 12.7 (length)×5.1 (width)×0.76(thickness) centimeter core of a phenolic/fiberglass honeycomb(available under the trade designation Nomex® from DuPont Fibers,Wilmington, Del.) having a 0.51 millimeter thick faceskin of phenolicresin impregnated fiberglass on each major surface. When the honeycombsubstrate was used, the faceskin surface was wiped clean with methylethyl ketone prior to coating with an aqueous adhesive dispersion.

One end of the substrate was covered with a 1.25 centimeter wide pieceof masking tape to provide a tab end for peel adhesion testing. Thesubstrate was held down by placing strips of masking tape along thelengthwise edges of the substrate such that between about 0.32 and 0.64centimeters of the edge was covered. The remaining exposed surface area,having a length of 11.4 centimeters and a width of between about 3.8 and4.5 centimeters, was then coated with the aqueous adhesive dispersionand dried in air at room temperature until no adhesive was transferredupon applying slight thumb pressure, typically about 20 to about 40minutes. The dried coating had a slightly hazy appearance and wasessentially nontacky to the touch.

A strip of a multilayer decorative laminate film, having a length of17.8 centimeters, a width of 2.5 centimeters and a total thickness of0.28 millimeters, was placed on the coated surface of the rigidsubstrate such that one end extended between about 3.8 and 5.1centimeters beyond one end of the coated substrate. The multilayerdecorative laminate film was obtained from the Boeing Company (Seattle,Wash.), and is described in the Boeing document BMS 5-127E, Section8.4.1.1.1(Revised Jun. 24, 1994). Briefly, the multilayer decorativelaminate film included two layers of poly(vinyl fluoride) film(available under the trade designation TEDLAR® PVF Film from DupontCompany, Wilmington, Del.) laminated to one major surface of a textureretention material. The opposite major surface of the texture retentionmaterial was bonded to the rigid substrate by use of the adhesivecomposition. A 2.5×10.2 centimeter silicone rubber heating blankethaving embedded therein heating elements connected to, and controlledby, a Watlow Model 985 programmable heater (commercially available fromWatlow Controls, Winona, Minn.) was placed over an aluminum plate havingthe same length and width and a thickness of 0.10 centimeters.Poly(tetrafluoroethylene) tape was wrapped around both the aluminumplate and silicone blanket to hold them together, and a thermocouple wasinserted between the aluminum plate and heating blanket. This tapedcombination was positioned on top of the multilayer decorative laminatefilm such that it covered the width of the decorative laminate film andabutted the taped tab. After clamping the entire assembly together using3 one inch metal clips it was heated from room temperature to 113° C.over a period of about 2 minutes and held at that temperature for 8minutes to cure the adhesive composition. The clips then were removed,the assembly taken apart, and the bonded structure of rigid substrateand multilayer decorative laminate film was allowed to cool to roomtemperature.

Method B

A core of Nomex® honeycomb covered with faceskins as described forMethod A above, and having a length and width of 30.5 centimeters each,was abraded on one of the faceskin surfaces using methyl ethyl ketoneand a 3M Heavy Duty Scotch-Brite™ Scouring Pad (commercially availablefrom 3M Company, St. Paul, Minn.) until the surface no longer exhibiteda glossy appearance.

Just prior to application, parts A and B of the aqueous adhesivedispersion, prepared as described in “General Preparation of AqueousAdhesive Dispersions—Method B”, were combined and mixed using anoverhead air motor as described therein until a uniform dispersion wasobtained, generally between about 10 and 15 minutes. Next, the aqueousadhesive dispersion of the invention was filtered through a No. 64 meshcone strainer filter (commercially available from Tufco Industries,Inc., Green Bay, Wis.) before spraying it, at various concentrations,onto the abraded faceskin surface of the rigid substrate. Spraying wasdone using a DeVILBISS Model JGHV-530 spraygun (commercially availablefrom DeVILBISS Ransberg Co., Maumee, Ohio) equipped with a #33A air cap.The air line pressure was 40 pounds/inch², which resulted in a pressureof 4 pounds/inch² at the exit orifice of the air cap. The spray tip washeld about 23+/−7.6 centimeters from the rigid substrate surface duringspraying. The sprayed substrate was dried in air at room temperatureuntil the adhesive coating appeared clear, typically about 15 minutes.The dried coating was very slightly tacky to the touch.

A strip of 3M™ 471 Vinyl Tape, having a width of 5.1 centimeters, wasplaced along the entire length of two opposing edges of the adhesivecoated faceskin surface so as to leave an exposed faceskin area of 30.5by 20.3 centimeters. This provided unbonded tab ends which could be usedin the evaluation of peel adhesion strength. The entire coated/tapedsurface of the rigid substrate was then covered with a piece of amultilayer decorative laminate film, having a length and width of 30.5centimeters. The multilayer decorative laminate film was obtained fromthe Boeing Company (Seattle, Wash.) and was as described above. Thisassembly was placed in a custom made (Greco Manufacturing, Buchanan,Mich.), scaled down version of a commercially available laminator (Model3X4-230-1-60, Greco Manufacturing, Buchanan, Mich.) equipped with asilicone rubber blanket, heat and vacuum sources, and having thedimensions of 94.0 by 81.3 centimeters. The custom made laminatoremployed radiant bar elements for heating instead of quartz halogenlamps as used in the commercial laminator. A vacuum of 635 millimetersHg was pulled and then heating from about 22° C. to 113° C. begun at arate of between 5.5 and 8.3° C./minute. After reaching 113° C. theassembly was held at that temperature for 8 minutes to substantiallycure the adhesive composition, then allowed to cool to about 71° C.before releasing the vacuum and removing the cured assembly. Barsmeasuring 2.5 by 30.5 centimeters were cut perpendicular to the tapeddirection so as to provide two unbonded tab ends on each bar, with abonded length of 20.3 centimeters centered between the tabs. After agingat various conditions the bars were cut to provide two specimens eachhaving a length of 7.6 centimeters including a 5.1 centimeter taped tabend. Each specimen was further treated in the following manner. Thevinyl tape was removed and the underlying faceskin layer, opposite theadhesive coated faceskin layer, and honeycomb core were cut away. Thespecimen thus obtained had a 5.1 centimeter long tab of faceskin and a7.6 centimeter long section of decorative laminate film, 2.5 centimetersof said decorative laminate film being bonded to the remaining faceskincovered honeycomb core and 5.1 centimeters being unbonded.

Test Methods

Evaluation of Drying Time

Drying time was evaluated using thermogravimetric analysis (TGA) tomeasure the amount of remaining solvent after a prescribed dryingprotocol. More specifically, about 20 milligrams of an aqueous blend ofa polyepoxide resin and either microspheres, or an analogous acryliclatex, was placed in a Seiko Model SSC/5200Thermogravimetric/Differential Thermal Analyzer (Seiko Instruments USA,Inc. Torrance, Calf.) and dried according to the following method.Beginning at 25° C., the sample was heated to 90° C. at 65° C./minute,held at 90° C. for 45 minutes, heated at 30° C./minute to 120° C., andheld at 120° C. for 20 minutes. Throughout the experiment the testchamber was purged with air at 400 milliliters/minute. The weight losswas monitored from the beginning to the end of the experiment. Theresults were normalized to 100% dryness, i.e., the constant weightobtained at the end of the 120° C. stage. Results are reported throughthe end of the 90° C. isotherm stage.

Evaluation of Effect of Microsphere Reactivity

The interaction of reactive groups on the microsphere with polyepoxideresins was evaluated by differential scanning calorimetry. Morespecifically, about 20 milligrams of a dried adhesive film was placed ina Seiko Model SSC/5200 Differential Scanning Calorimeter (SeikoInstruments USA, Inc.) and heated from 25° C. to 240° C. at a rate of20° C./minute under a nitrogen purge (30 millimeters/minute). The heatof reaction (i.e., exotherm) was plotted versus temperature.

Evaluation of Peel Adhesion Strength

Method A

The cured structure having a multilayer decorative laminate film bondedto a rigid substrate was evaluated for peel adhesion strength by takingthe tab end of the decorative laminate film between thumb and side offorefinger and peeling it back away from the rigid substrate at an angleof 180° and a rate of about 152 centimeters/minute. Two specimens weretested for each example. A grade of “Yes” was assigned to thosespecimens where there occurred tensile failure of the multilayerdecorative laminate film, i.e., the decorative laminate film itselftore. This result indicated that the bond strength of the adhesivecomposition to both the rigid substrate and the decorative laminate filmwas greater than the tensile strength of the decorative laminate film. Agrade of “No” was assigned to those examples where failure occurred ateither the adhesive/rigid substrate or the adhesive/decorative laminatefilm interface. Such results indicated that the tensile strength of thedecorative laminate film was greater than the bond strength of eitherthe adhesive to the rigid substrate or the bond strength of the adhesiveto the decorative laminate film.

Method B

The cured structure having a multilayer decorative laminate film bondedto a rigid substrate was evaluated for 180 degree angle peel adhesioncharacteristics using the test method described in ASTM D 903 with thefollowing modifications. Two specimens, each measuring 2.5 centimetersby 7.6 centimeters (including a 5.1 centimeter faceskin tab at one end),were peeled at a rate of 5.1 centimeters/minute. For each specimen theaverage peel strength was determined over a peel length of 2.5centimeters. The values reported are an average of two specimens. ASINTECH Model 10 Mechanical Tester (commercially available from SINTECH,Inc., Stoughton, Mass.) fitted with a 90.7 kilogram load cell was used.The specimens were mounted such that the 5.1 centimeter long faceskintab was held by a bottom static grip and 2.5 centimeters of thedecorative laminate film tab was held by a top dynamic grip. Themechanical-tester employed Testworks software, Version 1.2, to analyzethe data and provide results. Peel adhesion strength was measured aftervarious aging conditions, and is reported in Newtons/centimeter. (Peeladhesion strengths were measured in units of pounds/inch width andconverted into units of Newtons/centimeters by multiplying by 1.751).Preferably the multilayer decorative laminate film component tore ratherthan peeled away. In these cases the peel value at failure is reported.

Evaluation of Thermal Removability

Method A

The cured structure having a multilayer decorative laminate film bondedto a rigid substrate was evaluated for thermal removability in thefollowing manner. A sample, measuring 2.5 centimeters by 10.2centimeters, was first equilibrated at a temperature of 121° C. forbetween about 30 and about 60 minutes in a forced air oven, thenremoved, and (while still hot) the tab end of the decorative laminatefilm was taken between thumb and side of forefinger and peeled back awayfrom the rigid substrate at an angle of 180° and a rate of about 152centimeters/minute. A grade of “Yes” was assigned to those exampleswhere the cured adhesive and decorative laminate film were removedcleanly, or left only a minimum amount of adhesive residue on the rigidsubstrate. Any minimum residue remaining on the rigid substrate waseasily removed by scraping while still warm. A grade of “No” wasassigned to those examples where the cured adhesive and decorativelaminate film tore or left a significant amount of adhesive residue onthe rigid substrate that could not be easily removed by scraping whilestill warm.

Method B

The peel adhesion value of two test specimens, prepared as described in“General Preparation of Laminates—Method B”, was measured in a heatedtest chamber using the procedure described in “Evaluation of PeelAdhesion Strength—Method B” above. The specimens were equilibrated at121° C. for 5 minutes before testing.

Glossary

Various abbreviations are used in the following examples. Theabbreviations are defined as follows:

AA acrylic acid ACRYSOL ® SCT-275 a nonionic, hydrophobic polyethyleneoxide urethane, 17.5% solids in water:butyl carbitol/ 75:25 (w/w),commercially available from Rohm & Haas Company, Philadelphia, PA.ANTIBLAZE ® N a water soluble, cyclic phosphonate ester; commerciallyavailable from Albright & Wilson Americas, Inc., Glen Allen, VA.CUREZOL ® 2E4MZ 2-ethyl-4-methyl imidazole, commercially available fromAir Products and Chemicals, Allentown, PA. CUREZOL ® 2PHZ-S finelyground 2-phenyl-4,5-dihydroxymethyl imidazole, commercially availablefrom Air Products and Chemicals, Allentown, PA. DCA-2214,7,10-trioxadecane-1,13-diamine, commercially available from DixieChemical Company, Pasadena, TX. Imidazole commercially available fromAldrich Chemical Company, Inc., Milwaukee, WI. Epi-Rez ® 3520-WY-55 anaqueous dispersion of diglycidyl ether of bisphenol A, 53.5% solids,having an average epoxide equivalent weight of 535, available from ShellChemical Company, Houston, TX. Epi-Rez ® WD 510 a water dispersibleliquid diglycidyl ether of bisphenol A, 100% solids, having an averageepoxide equivalent weight of 200, available from Shell Chemical Company,Houston, TX. Epi-Rez ® 5522-WY-55 an aqueous dispersion of diglycidylether of bisphenol A, 55% solids, having an average epoxide equivalentweight of 625, available from Shell Chemical Company, Houston, TX. IOAisooctyl acrylate NCI 549802 4,7,10-trioxatridecane-1,13-diamine,commercially available from BASF Corporation, Mount Olive, N.J. PEOApoly(ethylene oxide) monoacrylate, average molecular weight of 750.

Unless noted otherwise, in the following examples all amounts are givenin grams rounded to the nearest hundreth.

Preparation of Microspheres

An aqueous suspension of solid microspheres was prepared for use in theadhesive composition. More specifically, 4.8 grams of acrylic acid (AA),2.4 grams of poly(ethylene oxide) monoacrylate (PEOA), and 1.13 gramsLucidol™ 70 (70% benzoyl peroxide, commercially available from ElfAtochem North America, Inc., Philadelphia, Pa.) were dissolved in 232grams isooctyl acrylate (IOA). Next, 0.75 grams of Siponate™ DS-10(sodium dodecyl benzene sulfonate surfactant, commercially availablefrom Rhone-Poulenc, Inc., Cranbury, N.J.) was dissolved in 360 grams ofwater. The isooctyl acrylate mixture was then added to the surfactantsolution and emulsified using an Omni™ Mixer homogenizer until thedroplet size was less than 5 micrometers. Typically this took about 10minutes or less. The suspension was then charged into a 1 liter baffledreactor, degassed with nitrogen, heated to 65° C., reacted at thistemperature for 8 hours before allowing to cool to room temperature. Thefinal suspension (40% solids) contained polymeric, elastomeric, solventinsoluble but solvent dispersible microspheres that were substantiallynonreactive and that had a composition of IOA:AA:PEOA/97:2:1 (w/w/w).This suspension of microspheres was used in the following examples.

Compositions 1-6

A series of aqueous adhesive dispersions was prepared as described abovein “General Preparation of Aqueous Dispersions of AdhesiveCompositions—Method A” with the following exceptions. No flame retardantwas employed in Compositions 1 or 2; in Compositions 1 and 3 thecurative was dispersed not dissolved; and in Composition 6 a mixture ofpolyepoxide dispersions were employed as indicated. The components andamounts are set forth in Table 1 below.

Comparative Composition 1

Comparative Composition 1 was formulated using a latex emulsion in placeof the microsphere suspension. The latex component had the samecomposition as the microsphere component (IOA:AA:PEOA/97:2:1 (w/w/w))which was described in “Preparation of Microspheres”. More specifically,750 grams of deionized water, 0.25 grams of sodium bicarbonate, and 8.0grams of Siponate™ DS-10 were added to a 2 liter glass reaction vesse, apurge of nitrogen gas begun and the mixture heated, using two 250 Wattinfrared heat lamps, to 35° C. with stirring at about 50revolutions/minute (rpm) to dissolve the Siponate™ DS-10. This typicallytook about 10 minutes. In a plastic beaker were mixed 242.5 grains ofIOA, 5.0 grams of AA and 2.5 grams of PEOA to give a slightly hazysolution. Next, the stirring rate in the glass flask was increased toabout 200 rpm and the solution was added rapidly in a single portion,all while maintaining a nitrogen purge. After mixing between about 2 to5 minutes, 1.0 gram of a 0.2% (w:w) aqueous solution of ferric sulfateheptahydrate and 0.8 grams of potassium persulfate solid were addedwhile continuing to stir with a nitrogen purge. After about 8 to about12 minutes an exotherm occurred resulting in a temperature increase toapproximately 45° C. over a period of about 32 minutes. The mixture wasthen heated with infrared heat lamps to a temperature of 60° C. and heldthere for 3 hours followed by cooling to between about 32° C. and 40° C.using a cold water bath while stirring to give a 25.7% solids latexemulsion having a composition of IOA:AA:PEOA/97:2:1 (w/w/w). This latexemulsion was then concentrated down to 43.4% solids by evaporation ofsome of the water using a hot plate with stirring.

This latex emulsion was used to prepare an aqueous adhesive compositionas described above in “General Preparation of Aqueous Dispersions ofAdhesive Compositions—Method A” but with the exception that the latexemulsion was used in place of the microsphere suspension. The componentsand amounts are set forth in Table 1 below.

TABLE 1 Epoxide Microsphere Thickener Epoxide:Acrylic Dry DispersionSuspension Curing Agent Solution Flame Retardant % Ratio C* (grams)(grams) I.D. (grams) (grams) (grams) Solids (wt.:wt.) 1 8.00 16.00CUREZOL ® 1.00 1.60 48.0 41:59 2PHZ 0.40 2 10.00 16.00 Imidazole 1.002.50 49.7 46:54 0.20 3 8.00 8.00 CUREZOL ® 1.00 1.20 50.4 58:42 2PHZ0.40 4 8.00 8.00 CUREZOL ® 0.50 0.55 49.5 58:42 2E4MZ + DCA- 221 0.20 +0.20 5 8.00 8.00 CUREZOL ® 1.00 0 46.5 58:42 2E4MZ 0.25 6 6.00 ± 2.00¹8.00 CUREZOL ® 0.50 0 53.2 62:38 2E4MZ + DCA- 221 0.20 + 0.20 CC 21.7020.00² CUREZOL ® 0.63 1.36 51.7 58:42 2E4MZ + DCA- 221 0.54 + 0.54 C* =Composition. CC = Comparative Composition. 1) a mixture of Epi-Rez ®3520-WY-55 dispersion and Epi-Rez ® WD510 liquid/6:2 (w:w). 2) 43.4%solids latex emulsion, not microsphere suspension.

EXAMPLES 1-9 AND COMPARATIVE EXAMPLE 1

The compositions described in Table 1 were used to coat a surface ofeither a rigid polycarbonate substrate (PC) or a faceskin coveredhoneycomb core (FS), then dried and laminated to a 0.28 millimeter thickmultilayer decorative laminate film as described in “General Preparationof Laminates—Method A”. The cured samples were evaluated for peeladhesion strength as given in “Evaluation of Peel AdhesionStrength—Method A” under two different conditions: 1) immediately aftercuring and cooling to room temperature (23° C.), and 2) immediatelyafter equilibrating for 60 minutes at 71° C. in a forced air oven (thesamples were removed and tested while still hot). Thermal removabilitywas also determined as described in “Evaluation of ThermalRemovability—Method A” above. The results are shown in Table 2.

TABLE 2 Epoxide: Acrylic Peel Adhesion Strength Dry (Tearing ofDecorative Thermally Ex. Ratio Laminate Film?) Removable @ No. SurfaceComposition (wt:wt) @ 23° C. @ 71° C. 121° C.? 1 PC 1 41:59 Yes N.T. Yes2 PC 2 46:54 Yes N.T. Yes 3 PC 3 58:42 Yes No Yes 4 PC 4 58:42 Yes N.T.Yes 5 PC 5 58:42 Yes Yes Yes 6 PC 6 62:38 Yes N.T. Yes 7 FS 4 58:42 YesYes Yes 8 FS 5 58:42 Yes N.T. No 9 FS 6 62:38 Yes N.T. No CE-1 FS CC58:42 No No No N.T. = Not Tested

Compositions 7 and 8

Two aqueous adhesive dispersions of the invention were prepared asdescribed above in “General Preparation of Aqueous Dispersions ofAdhesive Compositions—Method B”. The components and amounts are setforth in Table 3 below.

TABLE 3 Epoxide: Epoxide Microsphere Curing Thickener Flame AcrylicDispersion Suspension Agent I.D. Solution Retardant % Dry Ratio C*(grams) (grams) (grams) (grams) (grams) Solids (wt.:wt.) 7 537.0  537.0CUREZOL ® 14.9 33.5 49.9 58:42 2E4MZ + DCA-221 13.4 + 13.4 8 2153.41739.6 CUREZOL ® 68.0 134.8 50.8 64:36 2E4MZ + DCA-221 53.7 + 53.7 C*Composition.

EXAMPLE 10 AND COMPARATIVE EXAMPLE 2

Composition 7 described in Table 3 was coated onto a surface of afaceskin covered honeycomb core which was then dried and laminated to a0.28 millimeter thick multilayer decorative laminate film as describedin “General Preparation of Laminates—Method A”. The resulting curedlaminate was evaluated for peel adhesion strength and thermalremovability as given for Examples 1-6 above. The results, along withthose from Comparative Example 2, are shown in Table 4.

TABLE 4 Epoxide: Peel Adhesion Strength Acrylic (Tearing of DecorativeThermally Ex. Dry Ratio Laminate Film?) Removable No. Composition(wt.:wt.) @ 23 C. @ 71 C. @ 121 C.? 10 7 58:42 Yes Yes Yes CE-2 cc 58:42No No No

EXAMPLES 11 AND 12

Examples 11 and 12 were prepared by spraying Compositions 7 and 8 onto asurface of a faceskin covered honeycomb core, at a coating level of 37.7grams/meter², which was then dried and laminated to a 0.28 millimeterthick multilayer decorative laminate film as described in “GeneralPreparation of Laminates—Method B”. The resulting cured laminates wereevaluated immediately after preparation (0 days) and after standing for7 days at room temperature, for peel adhesion strength at roomtemperature (24° C.) and thermal removability according to “Evaluationof Peel Adhesion Strength—Method B” and “Evaluation of ThermalRemovability Method B”. The results are shown in Table 5.

TABLE 5 Peel Adhesion Strength Thermal (N/cm) Removability (N/cm) @ 24C. @ 24 C. @ 121 C. Ex. (Aged 0 (Aged 7 (Aged 0 (Aged 7 No. CompositionDays) Days) Days) Days) 11 7 4.4 7.9 0.3 0.7 12 8 7.2 8.9 1.0 1.6

The results in Table 5 show that a peel adhesion strength of at least4.4 Newtons/centimeter is provided by the adhesive compositions. Inaddition, upon heating to 121° C. the decorative laminate films andadhesive were cleanly removable, exhibiting peel adhesion values of 1.6Newtons/centimeter or less. For all specimens evaluated, an adhesivefailure mode was observed with the adhesive layer remaining on thedecorative laminate film.

EXAMPLES 13 AND 14

Examples 13 and 14 were prepared in the same manner described forExamples 11 and 12 respectively. The resulting cured laminates wereevaluated immediately after preparation (0 days) for peel adhesionstrength at room temperature (24° C.) and thermal removability. Inaddition, the cured laminates were aged at 71° C. for 7 days in a forcedair oven, cooled to room temperature, then evaluated for peel adhesionstrength (at both 24° C. and 71° C.) and thermal removability. Testingwas done as described for Examples 11 and 12. For peel adhesion at 71°C. the equilibration time at temperature was 5 minutes. The results areshown in Table 6.

TABLE 6 Peel Adhesion Strength Thermal (N/cm) Removability @ 24 C. @ 24C. @ 71 C. (N/cm) @ 121 C. Ex. Composi- (Aged 0 (Aged 7 (Aged 7 (Aged 0(Aged No. tion Days) Days) Days) Days) 7 Days) 13 7 4.4 17.5* 13.3** 0.33.0 14 8 7.2 20.1* 15.0* 1.0 3.1 *decorative laminate film tore; peelvalue is that at failure. **0.3 centimeter edges did not peel; peelvalues were calculated using 1.9 centimeter width.

The results in Table 6 show that a peel adhesion strength of at least4.4 Newtons/centimeter is provided by the adhesive compositions, andthat upon aging at 71° C. the peel adhesion strength was generallygreater than the strength of the decorative laminate film as evidencedby tearing of the decorative laminate film. In addition, when heated to121° C. the decorative laminate films and adhesive were cleanlyremovable, exhibiting peel adhesion values of 3.1 Newtons/centimeter orless. For those specimens in which the decorative laminate film did nottear, an adhesive failure mode was observed with the adhesive layerremaining on the decorative laminate film.

EXAMPLES 15 AND 16

Examples 15 and 16 were prepared in the same manner described forExamples 11 and 12 respectively. The resulting cured laminates were agedat 49° C. and 95% (+/−5%) relative humidity for 7 days and cooled toroom temperature. Specimens were evaluated immediately after preparation(0 days) for peel adhesion strength at room temperature (24° C.) andthermal removability. They were also evaluated after aging for peeladhesion strength at both 24° C. and 71° C., and thermal removability.Testing was done as described for Examples 11 and 12. For peel adhesionat 71° C. the equilibration time at temperature was 5 minutes. Theresults are shown in Table 7.

TABLE 7 Peel Adhesion Strength Thermal (N/cm) Removability @ 24 C. @ 24C. @ 71 C. (N/cm) @ 121 C. Ex. Composi- (Aged 0 (Aged 7 (Aged 7 (Aged 0(Aged No. tion Days) Days) Days) Days) 7 Days) 15 7 4.4 17.1* 6.5 0.31.9 16 8 7.2 23.2* 7.5 1.0 2.5 *decorative laminate film tore; peelvalue is that at failure.

The results in Table 7 show that a peel adhesion strength of at least4.4 Newtons/centimeter is provided by the adhesive compositions, andthat upon aging at 71° C. the peel adhesion strength was generallygreater than the strength of the decorative laminate film as evidencedby tearing of the decorative laminate film. In addition, when heated to121° C. the decorative laminate films and adhesive were cleanlyremovable, exhibiting peel adhesion values of 2.5 Newtons/centimeter orless. For those specimens in which the decorative laminate film did nottear, an adhesive failure mode was observed with the adhesive layerremaining on the decorative laminate film.

EXAMPLE 17 AND COMPARATIVE EXAMPLE 3

To evaluate the differences in drying times for an adhesive compositioncontaining a microsphere suspension and one having a latex emulsioncomprised of the same components as the microsphere suspensionthermogravimetric analysis was used to measure the rate of drying. Morespecifically, two aqueous blends were prepared. The first, representingan example of the invention, was a blend of Epi-Rez® 5522-WY-55 and amicrosphere suspension, prepared as described above in “GeneralPreparation of Aqueous Adhesive Dispersions—Method A”, in a wet weightratio of 50:50 which resulted in a 47% solids blend having apolyepoxide:microsphere dry ratio of 58:42 (w:w). The second,representing a comparative example, was a blend of Epi-Rez® 5522-WY-55and the latex emulsion of the Comparative Composition in a wet weightratio of 50:50 which resulted in a 49% solids blend having apolyepoxide:acrylic resin dry ratio of 56:44 (w:w). These were subjectedto thermogravimetric analysis as described in the test method“Evaluation of Drying Time”. The results are shown in Table 8 below.

TABLE 8 % Dry % Dry % Dry % Dry % Dry % Dry @ @ @ @ @ @ Ex. t = 0 t = 10t = 20 t = 30 t = 40 t = 46 No. min. min. min. min. min. min. 17 0.050.9 80.9 94.1 97.3 98.2 CE-3 0.0 26.5 39.3 46.4 51.3 54.0 Note: for t =0 to 1 minute the temperature was being ramped from 25° C. to 90° C.;for t = 1 to 46 minutes, the temperature was held at 90° C.

The results in Table 8 show that the microsphere-containing blend driesmore rapidly than a blend containing the analogous acrylic latex. Forexample, after a time of 20 minutes, the microsphere-containing blend isabout 80% dry while the comparative latex-containing blend is about 40%dry.

EXAMPLE 18 AND COMPARATIVE EXAMPLE 4

The effect of available reactive sites on the microsphere component wasevaluated by using Differential Scanning Calorimetry (DSC) to determineif the presence of such sites resulted in a reaction with thepolyepoxide resin Two different 1:1 (w:w) mixtures of Epi-Rez®3520-WY-55 and a microsphere suspension (40% solids) were prepared. Theresulting 47% solids mixtures had a polyepoxide resin:microsphere dryratio of 58:42 (w:w). The first mixture contained microspheres which hada composition of IOA:AA:PEOA/97:2:1 (w:w:w) (prepared as describedpreviously) and the second mixture contained microspheres which had acomposition of IOA:AA/98:2 (w:w) and was prepared in manner similar tothat used for the IOA:AA:PEOA/97:2:1 (w:w:w) composition. Each mixturewas coated onto a silicone treated release liner using a knife-over-bedcoater to give a wet thickness of 0.25 millimeters, and dried at 70° C.for 15 minutes to provide an adhesive film. These were evaluated forreactivity as described in the test method “Evaluation of Effect ofMicrosphere Reactivity”. The results are shown in Table 9 below.

TABLE 9 Ex. No. Reactive Microsphere Exotherm (Onset, ° C.) 28 No NoCE-5 Yes Yes (170)

The data in Table 9 indicate that the presence of a macromer component(PEOA) in the microsphere component results in the absence of adetectable exotherm, indicative of little or no reaction between themicrosphere component and the 1,2-epoxide groups of the polyepoxidecomponent.

Other embodiments are within the following claims. For example, theadhesive composition may be provided on a release liner such as bycoating and may subsequently be transferred from the release liner to asubstrate. The adhesive composition may also be in the form of a tape.

Also, instead of combining the polyepoxide resin, the curing agent andthe microspheres prior to coating the adhesive composition onto asubstrate, the curing agent may be first applied to the substrate andthen a composition that includes the polepoxide resin and microspheresmay be coated onto the curing agent. The entire composition may then becured, e.g., by heating.

The polyepoxide resin, the curing agent for the polyepoxide resin, themicrospheres and the relative amounts thereof can be selected such thatthe cured adhesive composition is cleanly thermally removable from apre-selected substrate, e.g., a specific one of two substrates.

The adhesive composition has been described as a polyepoxide resin basedadhesive composition, however, other thermosettable adhesivecompositions are contemplated. Examples of other thermosettable adhesivecompositions that may be made cleanly thermally removable attemperatures above their use temperatures by incorporating plurality ofmicrospheres therein include, e.g., polyurethanes, polyesters,structural acrylics, and cyanate esters.

What is claimed is:
 1. A thermosettable adhesive composition comprisinga polyepoxide resin; a curing agent; and a plurality of polymericmicrospheres, said microspheres, said polyepoxide resin, and said curingagent and the relative amounts thereof, being selected such that uponcure said cured composition forms a semi-structural bond to a substratethat is inseparable at use temperature and, when heated to a temperaturethat is greater than the use temperature, is cleanly removable from saidsubstrate.
 2. The composition of claim 1, wherein upon cure saidcomposition exhibits no greater than about 35% retention of initial peeladhesion strength at a temperature greater than the upper usetemperature.
 3. The composition of claim 1, wherein upon cure saidcomposition exhibits no greater than about 20% retention of initial peeladhesion strength at a temperature greater than the upper usetemperature.
 4. The composition of claim 1, wherein upon cure saidcomposition exhibits a peel adhesion strength of at least about 3.5 N/cmmeasured on an abraded phenolic resin impregnated fiberglass substrateat room temperature.
 5. The composition of claim 1, wherein upon curesaid composition exhibits a peel adhesion strength of at least about10.5 N/cm measured on an abraded phenolic resin impregnated fiberglasssubstrate at room temperature.
 6. The composition of claim 1, whereinupon cure said composition exhibits a peel adhesion strength of at leastabout 3.5 N/cm measured on a polycarbonate substrate at roomtemperature.
 7. The composition of claim 1, wherein upon cure saidcomposition exhibits a peel adhesion strength of at least about 10.5N/cm measured on a polycarbonate substrate at room temperature.
 8. Thecomposition of claim 1, wherein upon cure said composition exhibits apeel adhesion strength of at least 3.5 N/cm measured on an abradedphenolic resin impregnated fiberglass substrate at room temperature andno greater than about 35% retention of initial peel adhesion strength ata temperature greater than the upper use temperature.
 9. The compositionof claim 1, further comprising a flame retardant.
 10. The composition ofclaim 1, further comprising no greater than 20% by weight of a flameretardant.
 11. The composition of claim 1, wherein upon cure saidcomposition exhibits no greater than about 35% retention of initial peeladhesion strength at a temperature of greater than about 50° C.
 12. Thecomposition of claim 1, wherein upon cure said composition exhibits nogreater than about 35% retention of initial peel adhesion strength at atemperature of at least about 15° C. greater than said upper usetemperature.
 13. The composition of claim 1, wherein the ratio of weightof said polyepoxide resin to weight of said microspheres is betweenabout 70:30 and about 35:65.
 14. The composition of claim 1, wherein thecomposition cures at a temperature between about room temperature andabout 200° C.
 15. The composition of claim 1, wherein said microspherescomprise tacky microspheres.
 16. The composition of claim 1, whereinsaid microspheres comprise solid microspheres.
 17. The composition ofclaim 1, wherein said microspheres comprise hollow microspheres.
 18. Thecomposition of claim 1, wherein said microspheres comprise tack-freemicrospheres.
 19. The composition of claim 1, wherein said microspherescomprise tacky, solid microspheres.
 20. The composition of claim 1,wherein said microspheres have an average diameter between about 1micrometer and about 20 micrometers.
 21. The composition of claim 1,wherein said microspheres comprise the reaction product of isooctylacrylate, acrylic acid and poly(ethylene oxide)acrylate.
 22. Thecomposition of claim 1, wherein said composition is dispersed in water.23. The composition of claim 1, wherein said composition is tack-freeprior to cure.
 24. The composition of claim 1, wherein said curing agentcomprises a heat-activated curing agent.
 25. The composition of claim 1,wherein said curing agent comprises a photolytically-activatedcuringagent.
 26. The composition of claim 1, wherein said curing agentcomprises a blend of an epoxy homopolymerization catalyst and anaddition curing agent.
 27. The composition of claim 26, wherein saidcatalyst is selected from the group consisting of tertiary amines,imidazoles, substituted derivatives of imidazoles and combinationsthereof.
 28. The composition of claim 1, wherein upon cure saidcomposition is capable of forming a semi-structural bond to a phenolicresin impregnated fiberglass substrate.
 29. A thermosettable adhesivecomposition comprising between about 35 and about 70 parts by weight ofa polyepoxide resin; a curing agent; and between about 30 and about 65parts by weight polymeric microspheres, said composition, upon cure,forms a semi-structural bond to a substrate that is inseparable at usetemperature and, when heated to a temperature greater than the usetemperature, is cleanly removable from said substrate.
 30. Thecomposition of claim 29, wherein upon cure said composition exhibits apeel adhesion of at least about 3.5 N/cm measured on a polycarbonatesubstrate at room temperature, and no greater than about 35% retentionof initial peel adhesion strength at a temperature greater than theupper use temperature.
 31. A method for making a thermosettable adhesivecomposition cleanly removable by incorporating a plurality of polymericmicrospheres therein, said composition, upon cure, forms asemi-structural bond to a substrate that is inseparable at usetemperature and, when heated to a temperature greater than the usetemperature, is cleanly removable from said substrate.
 32. The method ofclaim 31, wherein upon cure said composition exhibits no greater thanabout 20% retention of initial peel adhesion strength at a temperaturegreater than the upper use temperature.
 33. The method of claim 31,wherein upon cure said composition exhibits a peel adhesion strength ofat least about 3.5 N/cm measured on an abraded phenolic resinimpregnated fiberglass substrate at room temperature and no greater thanabout 35% retention of initial peel adhesion strength at a temperaturegreater than the upper use temperature.